The present invention is directed to nanophotonic devices and, more particularly, to nanophotonic directional coupler devices.
Directional couplers transfer light signals from one waveguide to a second waveguide, without any direct contact of the two waveguides. The waveguides can be either photonic wire waveguides, such as that disclosed in U.S. Pat. No. 5,878,070, or photonic well waveguides, such as that disclosed in U.S. Pat. No. 5,790,583. U.S. Pat. Nos. 5,790,583 and 5,878,070 are incorporated by reference herein in their respective entireties. Specifically, energy is transferred from one waveguide to the other waveguide by optical tunnelingxe2x80x94a process of coherent coupling between the overlapping evanescent tails of the modes guided in each waveguide. Directional coupling is utilizable in wave division multiplexing (WDM) and dense wave division multiplexing (DWDM) applications, where light signals are selectively multiplexed and demultiplexed as needed.
Directional couplers are known in the prior art, such as lithium niobate couplers. Lithium niobate couplers, however, have a difference between the index of refraction inside of the waveguide and the index of refraction of the medium outside of the waveguide that is on the order of 0.01. As a result, a lithium niobate coupler must be formed to be at least several millimeters in length to achieve an acceptable level of signal transfer (i.e., transmission of the light signal from one waveguide to the other).
Semiconductor directional couplers have also been developed in the prior art which have lengths on the order of several hundred microns. The decrease in length from the lithium niobate designs is achieved due to the use of an index difference of up to 0.1 between the index of refraction of a medium inside the waveguide as compared to the index of refraction of a medium outside of the waveguide. In particular, directional couplers are used in connection with a microcavity resonator, such as that described in copending U.S. patent application Ser. No. 09/574,298, which consists of an oval shaped waveguide with arcuate ends, having small circumferences (typically 5 to 20 xcexcm), and two very small straight lengths. The straight sections of the oval waveguide are coupled to input and output waveguides, respectively, via essentially directional couplers with variable coupling factors. (The xe2x80x9ccoupling factorxe2x80x9d is the percentage of power that is coupled from the input waveguide into the resonator, and is determined by the length of the straight section and the gap separation.) Furthermore, in applications of resonators, it is desirable to be able to control the polarization content of the light that is coupled from the waveguide into the resonator, and vice versa. Hence, a very compact and polarization-controllable directional coupler is an important and integral part of the microcavity resonator device.
Thus there exists a need in the art for an optical device that overcomes the above-described shortcomings of the prior art.
A nanophotonic directional coupler device is provided which has a first waveguide and a second waveguide. Each waveguide has a respective input port and output port and coupling portion disposed therebetween. The coupling portion of the first waveguide is separated from the coupling portion of the second waveguide.
Preferably, the directional coupler is formed within the following parameters: a gap is defined between the waveguides that has a width which is less than 0.5 xcexcm; the width of the waveguides is less than 1 xcexcm; the length of the coupler (referred to as xe2x80x9ccoupler lengthxe2x80x9d) is less than 50 xcexcm; and a ratio of the index of refraction inside the waveguides to the index of refraction of the medium (e.g. air) in the gap between the waveguides is greater than 1.5. Additionally, the polarization of the light signal must be taken into consideration to ensure there is the desired level of transfer of the light signal between the waveguides.
It is preferred that symmetry be achieved in the directional coupler design. Specifically, the waveguides are to be identically or substantially identically formed (materials; dimensioning) to enable efficient transfer of the light signal. In a preferred embodiment, the waveguides are photonic well waveguides. On the other hand, if photonic wire waveguides are used, the same height in the core (the active medium through which the light propagates), in addition to the same width, is preferably used for both waveguides to enable efficient transfer of the light signal. Additionally, it is preferred that the height and width dimensions of the core be equal.
The operation of the directional coupler is affected by the polarization of the light signal. For transverse electric (TE) signals, it is preferred that the width of the waveguides be less than 0.25 xcexcm. As for transverse magnetic (TM) signals, it is preferred that the width of the waveguides be greater than 0.35 xcexcm. If a directional coupler is designed to accommodate a light signal of a certain polarization, it will not operate efficiently with a signal of a different polarization. For example, if a directional coupler is designed specifically to accommodate a TE light signal (be less than 0.25 xcexcm), a TM signal will pass through the directional coupler with little or no transfer of signal.
It is possible to form the directional coupler to be polarization independentxe2x80x94i.e., able to transfer light signal of either polarization. To form a xe2x80x9cuniversalxe2x80x9d directional coupler which is partly insensitive to polarization, the width of the waveguides may be formed greater than 0.25 xcexcm and less than 0.35 xcexcm. However, it should be noted that this xe2x80x9cuniversalxe2x80x9d design will not perform as well for each polarization as if the directional coupler was designed specifically for each polarization as described above. Moreover, a true polarization-independent directional coupler can be designed for certain specific parameters wherein the device is formed to transfer light with a transverse electric polarization at substantially the same power factor as light with transverse magnetic polarization. Although this design is limited to the design parameters, it is equally effective for both polarizations.
The above-described parameters affect the performance characteristics of the directional coupler. Theoretically, all of a light signal (100%) can transfer from one waveguide to the other in a directional coupler. Under actual conditions, there are losses and perfect xe2x80x9ccompletexe2x80x9d transfer of signal cannot be achieved. However, the various parameters described above can be adjusted to obtain different degrees of signal transfer.
Accordingly, it is an object of the subject invention to provide an improved directional coupler for transferring a light signal between two waveguides.
An additional object of the subject invention is provide a nanophotonic directional coupler which can be adapted to favor a polarization or be formed substantially polarization independent.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.